1. Field of the Invention
The invention relates to a pneumatic tool, more particularly to a pneumatic tool with trigger valve control.
2. Description of the Related Art
Referring to FIGS. 1 and 2, in U.S. Pat. No. 6,745,928, there is disclosed a conventional pneumatic tool 1 that includes a body 11, an air cylinder 12 provided in the body 11, a shuttle valve 13 sealing the air cylinder 12, a trigger valve unit 14 provided in the body 11 for controlling directions of air flow, an exhaust passage 15 formed in the body 11 and in fluid communication with the trigger valve unit 14 and the atmosphere, an intake passage 16 formed in the body 11 and in fluid communication with the trigger valve unit 14 and a compressed air source, and a vent passage 17 formed in the body 11. The trigger valve unit 14 includes a valve seat 141, a plunger 142 extending through the valve seat 141, a valve piston 143 mounted between the valve seat 141 and the plunger 142, and a plurality of O-rings 144 mounted among the valve seat 141, the plunger 142 and the valve piston 143.
By exerting an external force on the plunger 142 and due to the pressure difference, the positions of the plunger 142 and the valve piston 143 can be changed to allow air to flow from the intake passage 16 through the trigger valve unit 14 and the vent passage 17 so as to act on the shuttle valve 13 to allow compressed air to flow into the cylinder 12 for performing a fastener-driving operation. After completion of the fastener-driving operation, the shuttle valve 13 is biased to return to its original position, and the air acting on the shuttle valve 13 flows in a reverse direction into the atmosphere through the vent passage 17, the trigger valve unit 14 and the exhaust passage 15.
However, since the air pressure of the pneumatic tool 1 is rather high, and since the total length of the intake passage 16, the vent passage 17 and the exhaust passage 15 is too short to buffer effectively the airflow, when the fastener-driving operation is performed, a noise so loud as not to be ignored is generated. Moreover, since the valve piston 143 must be provide with three O-rings 144 to serve the purpose of controlling the air flow directions, not only the number of the O-rings 144 is too many, but also the associated friction resistance turns out rather high.